Design of VLSI Circuits in Software Encoding Code 3/9 in Software Design of VLSI Circuits

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Design of VLSI Circuits using software toattach uss code 39 in web,windows application Code 11 Warning example A digital IC w as found to malfunction when driven from its on-chip oscillator whereas it worked correctly from an external clock. This was because the slow ramps of the 32 kHz sine waveform from the oscillator a classical feedback loop built from a CMOS inverter and an o -chip crystal were corrupted by switching noise. The problem could have been avoided if a Schmitt trigger had been used instead of a normal bu er to shape the signal from the oscillator.

. Asynchronous r Software Code39 esets and gated clocks deserve special attention Noise is particularly critical on asynchronous reset signals because a spurious pulse could trigger reset of a circuit or of parts thereof at any time. To prevent this from happening, the voltage levels of reset signals are often made unsymmetric so as to maximize the noise margin for the reset s inactive state. This is why active-low resets in conjunction with unsymmetric TTL levels were generally preferred when supply voltage was 5 V.

A similar reasoning applies to critical inputs subject to impulse or transition signaling such as edge-triggered interrupt request lines and gated clocks. Unused inputs Do not leave any unused logic inputs open. Especially in MOS circuits, inputs may otherwise oat near the threshold voltage and unnecessarily draw DC.

In addition, an open input may pick up AC signals from a nearby source, switch in an unwanted way, draw even more supply current, and so contribute to overall switching noise. Mixed-signal design Analog signals are exposed to interference from the switching activities in the digital circuit blocks that coexist on the same die. While many of the countermeasures discussed so far such as geometric separation, distinct supply nets, and soft switching help to ght noise injection, let us see what else can be done on the digital side to minimize the impact.

Clocks have been found to be particularly pervasive polluters. The extremely fast clock ramps found necessary to minimize skew in edge-triggered designs in section 6.3.

1 result in strong harmonics. Two-phase level-sensitive clocking o ers an opportunity to relax slew rates because substantial skew can be accommodated by generously sizing the non-overlap phases. This helps one not only to manage with less aggressive clock waveforms, but also to get by with lighter clock bu ers, and to better distribute switching currents over the clock period.

Noise pollution from digital signals can further be reduced by resorting to CMOS current-mode logic (CML) families that rely on current switching in conjunction with reduced voltage swings [322][481]. Directing a constant current through either of two branches in a Y-topology network of n-channel MOSFETs reduces current spikes by two orders of magnitude over conventional CMOS [323], but brings about static power dissipation and extra routing overhead. Common series impedance coupling and crosstalk are not the only conveyors of noise when digital and analog subcircuits operate simultaneously on a common chip.

Coupling also occurs via the substrate as rapid voltage uctuations from the digital part tend to modulate MOSFET threshold voltages in the analog part via the body e ect. The impact is highly dependent on the substrate. 10.6 PROBLEMS being used (li Software barcode code39 ghtly vs. heavily doped; presence of an epitaxial layer; single, twin or triple wells). Please refer to [324] [325] [326] for advice on how to minimize substrate coupling.

Adopting di erential signaling on the analog side also helps because di erential circuits see most forms of interference as largely uncritical common mode noise signals. Noise analysis To compare ground bounce against noise margins, one can come up with a simpli ed noise-equivalent circuit and carry out electrical simulations using a SPICE-type software tool there. The equivalent circuit must include all signi cant polluters, the most vulnerable victims, plus the actual parasitics as extracted from the circuit s nal layout.

Estimating the combined e ects of ground bounce and crosstalk on signals in transit is much more demanding, though, as too many signals, layout parasitics, and data patterns are involved. In practice, specialized software tools are being used for the purposes of power grid analysis and crosstalk analysis, see sections 12.4.

7 and 12.4.8.

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